1. Field of the Invention
The present invention relates to a laminated-type piezoelectric element and an inkjet recording head having the same.
2. Description of the Related Art
Conventionally, a piezoelectric element has been used as a driving source (piezoelectric actuator) used for various types of devices with utilizing its characteristics of converting electric energy into mechanical displacement (deformation) by piezoelectric effects.
Of the piezoelectric elements, a piezoelectric element, which is formed into a laminated type in order to increase a deformation amount due to distortion, has the following structure. Individual electrodes and common electrodes are formed on one surface (one broad surface) of a piezoelectric sheet made of ceramic material such as PZT (lead zirconate titanate) by the screen printing method with using conductive material such as conductive paste. Piezoelectric sheets having the common electrodes and piezoelectric sheets having the individual electrodes are laminated alternately.
An on-demand type inkjet recording head has been proposed as an example of a device using the laminated-type piezoelectric element. As disclosed in U.S. Pat. No. 6,595,628, a cavity unit having a plurality of plates laminated includes a plurality of pressure chambers arranged in two parallel rows. A piezoelectric actuator serving as a laminated-type piezoelectric element having active portions (energy generating means) corresponding to each pressure chamber is joined to the cavity unit. Upon applying voltage to each active portion of the piezoelectric actuator, ink is selectively ejected from nozzles arranged in rows.
In this case, as disclosed in U.S. Pat. No. 6,595,628, the laminated-type piezoelectric element having two rows of the nozzles and active portions corresponding to the nozzles is formed by alternately laminating ceramic green sheets (piezoelectric sheets) made of piezoelectric material having individual electrodes on one surface thereof (broad surface) and other ceramic piezoelectric sheets having common electrodes on one surface thereof (broad surface), and then baking the laminated piezoelectric sheets.
The structure of the piezoelectric element disclosed in U.S. Pat. No. 6,595,628 will be described in more detail. As shown in FIG. 10, it is assumed that on a nozzle plate 100 having a shorter side in a first direction (X direction) and a longer side in a second direction (Y direction), rows of a plurality of nozzles 101 arranged in rows along the second direction (Y direction) are formed in tow rows in a zigzag manner while sandwiching a center line passing a center thereof in the short-side direction and extending in the second direction. The individual electrodes 103, which are formed on the broad surface (upper surface) of the lowermost piezoelectric sheets 102 in a laminated direction and on the broad surfaces of the odd-numbered sheets 102 of the piezoelectric sheets as counted from the lowermost piezoelectric sheets 102 upwardly, have a rectangle shape in plan view, which elongates in the first direction and is narrow in the second direction. The individual electrodes 103 are arranged in a zigzag manner in the second direction at the same intervals as the nozzles 101. One edges 103a of the individual electrodes 103 are located on both sides of the central line of the piezoelectric sheet 102. The other edges of each individual electrodes 103 in the X direction extend to an edge of a longer side of the piezoelectric sheet 102. It is assumed that each individual electrode 103 has a length L1 in the longitudinal direction Also, a long dummy common electrode 104 is formed in the vicinity of an edge of a short side of each piezoelectric sheet 102 along the short side.
Meanwhile, as shown in FIG. 10, a common electrode 106 formed on a broad surface (upper surface) of each of the even-numbered piezoelectric sheets 105 in the laminated direction has a length L2 in the short side direction (first direction) of the piezoelectric sheet 15, on a center side. The common electrode 106 has a band shape extending in the second direction. Each of long sides 106a, 106a on both sides of the common electrode 106 are distant by a length L3 from an edge of the long side of the piezoelectric sheet 105. Also, extraction portions 107 extending along an edge of the short side of the piezoelectric sheet 105 are integrally formed with the common electrode 106 in the vicinity of the short side. Also, on both sides of the common electrode 106, dummy individual electrodes 109 are located apart from the sides 106a and 106a appropriately and extend to the edge of each long side of the piezoelectric sheet 105.
When the piezoelectric sheets 102 and 105 are laminated alternately, each dummy individual electrode 109 partially overlaps each individual electrode 103 corresponding thereto as viewed in a plan view in the laminated direction. Likewise, the dummy common electrode 104 is also located to overlap the extraction portion 107.
When the plurality of piezoelectric sheets 102 and 105 are laminated alternately, a portion where each individual electrode 103 and each common electrode 106 overlap each other in the laminated direction defines an active portion. Meanwhile, through-holes 110 for electrically connecting the individual electrodes 103 each other in the laminated direction via electrically conductive materials are defined through the piezoelectric sheets 102 except for the lowermost one, at positions of the individual electrode 103 and dummy individual electrode 109 in the vicinity of the long side edge of each piezoelectric sheet 102. Similarly, through-holes 111 for electrically connecting the common electrodes 106 each other via electrically conductive materials are defined through the piezoelectric sheets 105 at positions of the extraction portion 107 and the dummy common electrode 104 in the vicinity of the short side of each piezoelectric sheet 105.
Each of the individual electrodes 103, the common electrodes 106, the dummy individual electrodes 109, and the dummy common electrodes 104 are formed by applying electrically conductive paste of, e.g., Ag—Pd (silver-palladium) system by the screen printing method to the piezoelectric sheets (green sheets) 102 and 105, and then baking them. Accordingly, as shown in FIG. 11, the length L4 of each active portion is equal to a projected length between a long side 106a of the band-shape common electrode 106 and the one side 103a of the individual electrode 103 located on upper or lower side of the piezoelectric sheet 102(105) interposed therebetween. That is, the length of each active portion is determined by the one side 103a of the individual electrode 103 and the long side 106a of the common electrode 106 with the piezoelectric sheets 102(105) interposed therebetween.